Note: Descriptions are shown in the official language in which they were submitted.
CA 02455840 2004-O1-27
INJECTION MOLDING UNIT WITH A DEVICE FOR MIXING AND METERING
PLASTIC MELT AS WELL AS ADDITIVES
The invention relates to an injection unit of an injection molding machine
with an
injection device which is configured as plunger/screw and operated in a
housing
for axial as well as rotating movements and which is provided with a backflow
prevention assembly, wherein the injection device is provided for plastication
during transport of the plastic granulate and acts as plunger, when the
plastic
melt is pressed through a nozzle into an injection mold, and a device for
mixing
and metering the plastic melt as well as additives such a colorants, foaming
agents, lubricants, or the like.
The state of the art, not supported here by printed publication, discloses,
e.g.,
injection units for injection molding machines in which the additives are
already
admixed into the plastic granulate ahead of the screw.
It is also known to provide conventional plunger-type screw injection molding
units with separate metering and mixing devices. These systems suffer,
however,
major problems as far as a permanent sealing of the shaft and plunger
passageways are concerned because jointed areas with relative movements
must be sealed against escape of melt.
On the basis of the prior art mentioned last, it is the object of the
invention to
provide a novel injection unit of an injection molding machine with an
injection
device which is configured as plunger/screw and operated in a housing for
axial
as well as rotating movements, and in which a separate metering and mixing
device is integrated for metered additive supply, without need to seal moving
parts against the outside to prevent escape of melt. Furthermore, it is the
object
of the invention to simplify the structure and to reduce the overall
structural size
of the device.
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This object is attained by the features of claim 1, in particular by the
characterizing part, whereby a spacer element is disposed between the backflow
prevention assembly of the screw and the plunger and has a length which
corresponds at least to the stroke of the injection unit, with the spacer
element
having a flow channel for ensuring in each position a fluidic communication
with a
further flow channel which initially extends radially outwards and then
parallel to
the movement and guide area of the plunger to an area between nozzle and
plunger, wherein the flow channel extends through the mixing and metering
device arranged adjacent to the movement and guide area of the plunger.
The device according to the invention has numerous advantages compared to
the state of the art. On the one hand, no moving parts have to be sealed from
outside against escape of melt, so that the construction is simplified. In
addition,
the coaxial configuration of the mixing and metering device results in a
straight
force flux while the structural size is reduced at the same time.
The mixing and metering device can advantageously embrace the plunger at
least partially in a circular ring shaped manner, wherein it is basically
possible in
this context to motionally connect the mixing and metering device in relation
to its
rotation movement with the injection device.
The last-mentioned solution advantageously eliminates the need for an
additional
drive for the mixing and metering device so that overall the device becomes
simpler.
According to a particular embodiment of the invention, the mixing and metering
device includes mixing elements which are arranged on a rotatable tube, which
is
rotationally connected with the plunger, and on the inner diameter of the
housing,
as well as at least an additive inlet. The mixing and metering device is
hereby
disposed in immediate proximity of the movement path of the plunger, but
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separate therefrom, thereby positively affecting the structural size, whereby
the
flow channel for the melt extends through the mixing and metering device about
the movement space of the plunger.
In such a mixing and metering device, the mixing elements may have, for
example, different surface configurations in opposite relationship, in
particular
toothed surfaces, to realize a thorough mixing of the melt with the additives
that
have been added to the mixing and metering device.
According to a further embodiment of the invention, the spacer element is
formed
by a continuously tapered shaft, whereby the circular ring shaped flow cross
section, which surrounds the shaft, is fully available for transport of the
melt from
the screw to the mixing and metering device. in a most simple manner, the
necessary movement clearance between plunger and screw can be realized,
without interrupting the flow channel for the melt, regardless of the position
of the
injection device.
Finally, a preferred embodiment of the invention is so configured that an
additional intermediate component is disposed anteriorly of the movement space
of the plunger between nozzle component and housing component and provides
an expanded accumulation chamber for the melt in x-direction, wherein the
plunger is provided on the nozzle side with a tapered shaft which provides the
expulsion of the melt.
Also this embodiment has the advantage, that the "first in - first out"
principle is
maintained because the melt components that enter first the accumulation
chamber are ultimately injected first through the nozzle into the injection
mold.
It would also be conceivable in accordance with a further embodiment of the
invention to implement the spacer element in form of a tapered shaft having a
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cross section which slightly narrows conically from the screw to the plunger,
wherein an aperture ring is disposed at slight distance to the movement zone
of
the plunger in the housing to reduce the circular ring shaped cross section,
and
wherein the shaft is provided with at least one throughbore, which extends in
axial direction and connects a flow channel, which extends out from the screw,
with a ring-shaped constriction of the shaft on the plunger side, for
establishing a
fluidic communication between the throughbore and the circular ring shaped
flow
cross section.
This particular solution has the advantage that no flow dead spaces are
generated in the flow cross section available for the melt, even when the
plunger/screw arrangement (injection device) has moved forward, so that the
so-called "first in - first out" principle is maintained also here.
Lastly, the invention relates also to an embodiment in which an additional
housing component having a nozzle is disposed ahead of the movement space
of the plunger and constitutes an expanded accumulation chamber for the melt,
whereby the plunger is provided on the nozzle side with a tapered shaft for
expelling the melt.
Also in this case, the principle of "first in - first out" is ultimately
maintained
because the melt components, which enter first the area anteriorly of the
plunger
from the flow channel extending through the mixing and metering device, enter
also first the injection mold during later expulsion of the melt, whereby the
subsequently melted melt components follow in the respective chronological
sequence.
Further advantages of the invention can be appreciated from the following
description of several exemplary embodiments. It is shown in:
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Fig. 1 a longitudinal section through a first embodiment of an injection unit
after expulsion of the melt;
Fig. 2 a longitudinal section through the injection unit according to Fig. 1
during charging of the accumulation chamber for the melt disposed in front of
the
plunger;
Fig. 3 a longitudinal section through the injection unit according to Figs. 1
and 2 with filled accumulation chamber for the melt;
Fig. 4 a longitudinal section through a second embodiment of an injection
unit after expulsion of the melt;
Fig. 5 a longitudinal section through the injection unit according to Fig. 4
during charging of the accumulation chamber for the melt;
Fig. 6 a longitudinal section through the injection unit according to Figs. 4
and 5 with filled accumulation chamber for the melt;
Fig. 7 a longitudinal section through a third embodiment of an injection
unit with anterior housing part;
Fig. 8 a longitudinal section through the injection unit according to Fig. 7
during charging of the accumulation chamber for the melt; and
Fig. 9 a longitudinal section through the injection unit according to Figs. 7
and 8 with filled accumulation chamber for the melt.
Reference numeral 10 designates in the drawings in general an injection unit
of
an injection molding machine.
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The injection unit 10 includes a housing 11 which is comprised of the
components 12 to 16.
Disposed in the components 12 and 13 is a circular cylindrical guide channel
17
in which a screw 18 of an injection device S is arranged rotatably and axially
moveably. The screw 18 is provided on its free end with a thrust device in a
manner not shown. Disposed on the other end of the screw 18 is a backflow
prevention assembly 19 followed by a tapered shaft 20 as well as a plunger 21.
The plunger 21 is arranged for longitudinal movement in a concomitantly
rotating
tube 22 having an outer circumferential surface for arrangement of alternating
circular ring shaped mixing elements 23 of a mixing and metering device 24.
Further mixing elements 25 are secured to the component 15 of the housing 11
at a distance and in confronting relation to the mixing elements 23 such that
a
circumferential circular ring shaped flow channel 26 is formed. The unitary
structure of plunger 21, tube 22 as well as mixing elements 23 is firmly or
form-fittingly connected with the tapered shaft as well as the screw 18 and
rotatably supported via ring bearings 27 within the housing 11.
Finally, the mixing and metering device 24 further includes devices for
additive
introduction 28 which are arranged within the component 15 and have an outlet
which terminates in the flow channel 26 of the mixing and metering device 24.
The flow channel 26 of the mixing and metering device 24 is connected via
radial
flow channel zones 29 and 30 with an accumulation chamber 31 provided for the
melt and disposed anteriorly of the plunger, on the one hand, and with an
annulus-forming flow channel 32 in surrounding relationship to the tapered
shaft 20.
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Lastly, the component 16 of the housing includes a nozzle bore 33 via which
melt
contained in the accumulation chamber 31 can be injected into a not shown
injection mold which is arranged anteriorly of the component 16.
Fig. 1 shows the position of the injection unit 10 and the injection device S
immediately after the injection process. It can be seen that the injection
device S
has been advanced by a maximum stroke H in the direction x to a position
before
the nozzle bore 33 (see comparison between Figs. 1 and 3). As a consequence,
the melt is pushed almost entirely out of the accumulation chamber 31 into the
not shown injection mold. When the screw 18 begins to rotate again, material
is
increasingly plasticized in the screw 18 and transported via the backflow
prevention assembly 19 into the annulus-forming flow channel 32 and further
via
the flow channel zones 30, 26 and 29 into the accumulation chamber 31 for the
melt. As the mixing and metering device 24 is rotationally connected with the
plunger/screw drive (drive of the injection device S), the melt is thoroughly
mixed
during transport of the melt while additives are supplied simultaneously,
before
the melt enters the accumulation chamber 31.
Since melt is increasingly transported into the accumulation chamber 31, the
injection device S, comprised of the plunger 21 of the tapered shaft 20 as
well as
the screw 18, is moved backwards in y-direction at the same time. This
procedure is only terminated when the accumulation chamber 31 is filled and
the
plunger impacts against a shoulder 34 during its movement in y-direction. This
is
the moment when again the not shown thrust device of the injection device S is
moved in x-direction, whereby the backflow prevention assembly 19 prevents a
backflow of the melt into the screw 18 during the injection process.
Figs. 4 to 6 illustrate another embodiment of the injection unit 10. The
differences
relate essentially only to the spacer element which is arranged between the
plunger 20 and the screw 18 and in this case is also implemented in the form
of a
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tapered shaft 35 which, however, converges in slightly conical manner from the
backflow prevention assembly 19 to the plunger 21. This shaft 35 has a central
throughbore 36 which begins in the area of the backflow prevention assembly 19
and ends in a ring-shaped constriction 37of the shaft 35 at the plunger-side
end
of the shaft 35 for establishing a fluidic communication between the
throughbore
36 and an annulus-forming flow channel 38.
Further arranged at a slight distance to the movement zone of the plunger 21
in
the housing component 13 is an aperture ring 39 which reduces the flow cross
section of the annulus-forming flow channel 32. Established between this
aperture ring 39 and a surface of the shaft 35 during movement of the
injection
device is a maximum gap Smax, when the accumulation chamber 31 is filled (end
position in y-direction, see Fig. 6), and a minimum gap sm;~, when the
accumulation chamber 31 is emptied (end position in x-direction, see Fig. 4).
This particular arrangement prevents flow dead spaces in each position of the
injection device S by increasing the flow resistance of the annulus-forming
flow
channel 32, as the injection device S progressively moves in x-direction, due
to
the continuous narrowing of the gap s, so that the actually longer flow path
through the throughbore 36, the annular constriction 37 as well as the return
path
(in y-direction) through the annulus-forming flow channel 32 to the flow
channel 30 still has a similar flow resistance as the afore-mentioned shorter
flow
path. In this way, melt that exits first the backflow prevention assembly 19
also
flows first through the flow channel 30 and ultimately reaches first the
accumulation chamber 31 so as to be pushed first through the nozzle bore into
the injection mold (first in - first out principle).
The individual position of this aforedescribed injection unit 10 are
illustrated in
Figs. 4 to 6.
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Finally, Figs. 7 to 9 illustrate a further embodiment of an injection unit 10
in which
an intermediate component 40 is arranged between the component 16, which
includes a nozzle bore 33, and the component 15 of the housing, for realizing
a
substantial prolongation of the accumulation chamber 31 in x-direction.
Furthermore, the plunger 21 is provided with a shaft-like ejection element 41
which has a smaller diameter than the plunger 21 so that a ring space 42 is
left
as flow cross section for the melt. After expulsion of the melt from the
accumulation chamber 31 (see Fig. 7), the injection device S is caused to move
again, just like in the other embodiments, to start a rotating movement of the
injection device S. At the same time, melt is transported via the various
afore-mentioned flow channels 32, 30, 26 and 29 to the ring space 42 and then
into the accumulation chamber 31 between nozzle bore 33 and a free end 43 of
the shaft 41. At the same time, the entire injection device S is moved
continuously backwards in y-direction, as a consequence of the increased
accumulation of melt in the accumulation chamber 31, until the end position in
y-
direction, shown in FIG. 9, has been reached. The afore-mentioned "first in -
first
out" principle is maintained in an advantageous manner also by this injection
unit
10 with anterior intermediate component.
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